The invention relates generally to methods and means of growing crystals, and moe particularly to methods and means of growing crystals of intermetallic compounds having as one of their components a metal selected from the group consisting of the lanthanide rare earth elements, and yttrium (hereinafter referred to as "intermetallic compounds of rare earth metals"), or alloys of said compounds, one with the other.
In the field of crystal growth, the need for improved control of chemical purity, homogeneity and crystallographic perfection have long been recognized. For example, single crystals of such well-known semiconductor materials as silicon and germanium which do not possess sufficient degrees of chemical purity, homogeneity or crystallographic perfection cannot be fabricated into useful devices. References hereinafter to crystallographic quality or the use of terms of description of relative crystallographic quality are to be understood in the sense used in the fields of x-ray or neutron diffraction.
Some of the most important problems in growing a crystal having desirable degrees of chemical purity, homogeneity and crystallographic perfection from a molten mass relate to the ease, accuracy and reproducibility of the control of the melt temperature, and of the uniformity of temperature in the region of the melt where crystal growth occurs.
A number of additional problems relate with particularity to the instance where the melt and the crystal to be grown contain a metal selected from the group consisting of the lanthanide rare earth elements, and yttrium. Some of the problems encountered are:
THE CHEMICAL REACTIVITY OF THE MELT, WHICH IS MANIFESTED BY ATTACK OF THE MELT UPON VIRTUALLY ANY MATERIAL USED AS A CRUCIBLE WHICH IS ALLOWED TO APPROACH THE TEMPERATURE OF THE MELT, AND BY CONTAMINATION OF THE MELT UPON CONTACT WITH THE ELEMENTS OXYGEN, NITROGEN, HYDROGEN OR CARBON, AMONG OTHERS; THE TENACIOUS NATURE OF THE DROSS WHICH FREQUENTLY APPEARS ON THE SURFACE OF SUCH MELTS, WITH THE CONSEQUENCE THAT RANDOM NUCLEATION OF SECOND GRAINS MAY BE EFFECTED; IN ADDITION, THE DROSS IS MORE THERMALLY EMISSIVE AND HAS HIGHER SURFACE TENSION THAN THE MOLTEN METAL, WHICH LEADS TO UNWANTED VARIATIONS IN TEMPERATURE IN THOSE LOCATIONS WHERE DROSS IS FOUND, WITH ASSOCIATED UNCONTROLLED VARIATIONS IN THE QUALITY OF THE CRYSTALS GROWN;
THE DEARTH OF RELIABLE DATA AS TO THE PHASE RELATIONS, MELTING POINTS AND OTHER THERMODYNAMIC CHARACTERISTICS OF INTERMETALLIC COMPOUNDS OF RARE EARTH METALS, THERE FREQUENTLY BEING SEVERAL PUBLICATIONS ON A GIVEN MATERIAL STATING MUTUALLY CONTRADICTORY FINDINGS OR RESULTS; THE OBSERVATION OF ANOMALOUS THERMAL EXPANSION EFFECTS IN SOME OF THE INTERMETALLIC COMPOUNDS OF RARE EARTH METALS, WITH THE CONSEQUENCES THAT CRYSTALLOGRAPHIC DAMAGE CAN RESULT TO GROWING OR GROWN CRYSTALS WHICH ARE PHYSICALLY CONSTRAINED IN A CONTAINER WHILE THE TEMPERATURE IS CHANGED, THUS INTRODUCING STRESSES OR STRAINS, AND THAT THE RANDOM NUCLEATION OF SECOND GRAINS AT THE INTERFACE BETWEEN THE GROWING CRYSTAL AND THE MELT MAY RESULT WHEN A CONSTRAINT IS PRESENT AT THE INTERFACE;
THE INTERMETALLIC COMPOUNDS OF RARE EARTH METALS ARE ALSO FREQUENTLY BRITTLE WHEN IN POLYCRYSTALLINE FORM AT ROOM TEMPERATURE, AND PRONE TO SHATTERING VIOLENTLY WHEN HEATED ABRUPTLY;
AND, THE INTERMETALLIC COMPOUNDS OF RARE EARTH METALS ARE READILY CONTAMINATED BY EXPOSURE TO OXYGEN, EVEN AT ROOM TEMPERATURE, WHEN IN FORMS HAVING LARGE SURFACE TO VOLUME RATIOS, FOR EXAMPLE, POWDERS OR GRANULES.
The prior art consists of a number of methods which have been employed with varying success for the provision of single crystals of intermetallic compounds wherein at least one constituent is a lanthanide rare earth metal. Also to be found in the prior art are methods which have been employed to provide for the preparation of crystals in an electric arc powered crystal growing apparatus. These methods include:
directionally freezing a melt contained in crucibles constructed from materials such as tantalum, aluminum oxide and boron nitride, the crucibles being held at the same temperature as the contained material;
the horizontal floating zone technique;
crystal pulling from a melt contained in a crucible maintained at the temperature of the melt;
the thermal imaging growth technique;
the method and means described by Loung in U.S. Pat. No. 3,160,497, for pulling ingots of refractory metals;
the method and means described by Reed et al in U.S. Pat. No. 3,625,660 for growing crystals of semiconductors;
and the method and means described by Rudness et al in U.S. Pat. No. 3,314,769, for growing crystals of metals.
Each of these methods, while yielding crystals of varying quality, suffers from one or more of the problems enumerated above, as will be presently outlined. The present invention overcomes each of the problems, as will become apparent hereinafter.
The method of directional freezing suffers from the problems of melt contamination, stresses and strain induced in the crystals, and nucleation of second grains. In extreme cases, the crucible has been known to fail entirely.
The horizontal floating zone technique suffers from the problems of stresses and strains induced in the grown crystal, and nucleation of second grains.
The method of crystal pulling from a melt contained in a crucible maintained at the temperature of the melt suffers from the problems of attack by the melt on the crucible and melt contamination.
The thermal imaging growth technique, although novel, suffers from the problem of temperature control, in that any material which might volatilize from the melt can coat the transparent envelope through which power is optically conveyed to the melt, thus resulting in changes of power at the melt which an operator may not be able to control. In an extreme case, the transparent envelope may be rendered opaque by deposits of such volatile material, halting operation by this method completely.
The method and means of Loung and of Reed et al make no reference whatever to the provision of a method or means for physically rotating the solid portion of the material which is intended to be melted with respect to the heat source. The method and means of Rudness et al suffers from the problem that powders of the intermetallic compounds of rare earth metals are oxidized by only traces of oxygen, so that this embodiment of their method and means would probably fail to provide acceptable crystals. In the embodiment wherein a solid feed rod is melted, the brittleness of the starting polycrystalline materials might lead to failure of the method. Furthermore, the technique of Rudness, et al, specifically does not allow for the independent rotation of the growing boule and the molten region from which it grows, thus not permitting to occur that mixing or stirring action which may be obtained by this means. Lastly, this technique demands that the crystal is the only solid area where the dross which frequently is present might collect, which would seriously damage the quality of the material grown.
Thus, the patented prior art referred to suffers from problems related to temperature control and homogeneity. As will be made apparent hereinafter, these problems are overcome by the present invention.